The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, all color videos are currently input and output in the RGB format. In other words, all colors may be expressed with colors of Red (R), Green (B), and Blue (B). However, the RGB format has a high correlation between respective color channels, so the compression ratio is low when a video is encoded in the RGB format. Accordingly, a general and common application currently uses a video format of the YCbCr format, not the RGB format, in storage, transmission, and compression of a video. A method of transforming the RGB format to the YCbCr format is defined in an international standard group, such as the International Telecommunication Union (ITU) or the Society of Motion Picture and Television Engineers (SMPTE). In the YCbCr, Y refers to a luminance component and Cb and Cr refer to chrominance components, and the correlation between respective color channels is substantially removed.
Most of the common applications currently use a signal in the 4:2:0 format, as well as the simply transformed YCbCr format. FIG. 1 is a diagram illustrating the YCbCr 4:4:4 format, FIG. 2 is a diagram illustrating the YCbCr 4:4:2 format, and FIG. 3 is a diagram illustrating the YCbCr 4:2:0 format. According to the YCbCr 4:2:0 format, information on chrominance signals, i.e. Cb and Cr, is transversely and longitudinally sub-sampled by ½, so that the information on the chrominance signals is decreased to ¼ as illustrated in FIG. 3. This uses a fact that a person is more sensitive to a luminescence signal than a chrominance signal. Accordingly, most of the current video codecs including MPEG-2/4, H.263, and H.264/MPEG-4 AVC basically encode and decode an input video in the YCbCr 4:2:0 format.
However, in this case, a loss of the chrominance signal of an encoded image is greatly generated compared to an original image. Accordingly, a professional application field, such as a digital cinema, a medical image, and a Ultra High Definition Television (UHDTV), uses the RGB 4:4:4 format or the YCbCr 4:4:4 format, not the YCbCr 4:2:0 format.
In order to support the format, H.264/AVC AMD supports a signal processing in an RGB area with high 4:4:4 intra/predictive profiles, and includes two support methods below. The first method is a common mode method of commonly applying an intra/inter mode, which has been determined at the time of encoding of a green chrominance signal, to a blue and a red in the processing of an RGB signal. The second method is an independent mode method of independently processing each of R, G, and B. In this case, as described above, the compression ratio of an encoded image is deteriorated due to the high correlation between the R, G, and B.
Accordingly, the high correlation between chrominance signals fundamentally exists in the RGB area, so that a research for improving the efficiency of an encoder through the removal of the correlation has been conducted.
Document 1 discloses a method, in which R and B signals are predicted using a G signal based on the fact that the linear relation is represented between R, G, and B signals. An inclination value and an offset value in a linear model are transmitted from an encoder to a decoder, and the decoder predicts the R signal and the B signal by using the G signal based on the transmitted inclination and offset values. Such a method may improve the prediction efficiency, but it is necessary to transmit an inclination value and an offset value for each macro block, so a quantity of side information is increased, causing the performance of the method to be limited.
In order to solve the limitation, Document 2 discloses a method, in which an inclination value and an offset value are estimated in already reconstructed G, R, and B signals for each block, so that it is not necessary to to transmit the inclination value and the offset value. That is, in the method, a decoder estimates an inclination and an offset based on values of samples of a left side and an upper side of an image, so that the encoding efficiency is improved through removal of side information for notifying of the correlation between channels.
Further, Document 3 discloses a researched method of applying a high weight value to a similar pixel when the similar pixel is positioned in a decoding completed adjacent area and a low weight value to an adjacent pixel having a low similarity in generating a currently estimated prediction image of B and R signals. In this case, a reference for determining a similarity between an image to be estimated and a decoding completed image is an already encoding completed G signal.    [Document 1] Byung Cheol Song, Yun Gu Lee, and Nak Hoon Kim“Block Adaptive Inter-Color Compensation Algorithm for RGB 4:4:4 Video Coding,” IEEE CVST., vol. 18, no. 10, pp. 1447-1451, October, 2008.    [Document 2] Y.-H. Kim, S.-Y. Jung, B. H. Choi and J. K. Park, “High Fidelity RGB Video coding Using Adaptive Inter-Plane Weighted Prediction,” IEEE CVST., vol. 19, No. 7, pp 1051-1056, July, 2009.    [Document 3] S. H. Lee, J. W. Moon, J. W. Byun and N. I. Cho, “A New Intra Prediction Method Using Channel Correlations for The H.264/AVC Intra Coding,” Picture coding Symposium 2009. March, 2009.
As described above, the existing researches use an encoded completed green signal for generation of a prediction image for encoding blue and red signals, to obtain an encoding gain.
However, in a case where there are various boundaries and colors inside an image to be estimated and there are also various colors and boundaries in an adjacent area, when a parameter having a minimum error is extracted using all adjacent pixels as described in the existing methods, an incorrect pixel may be used for the estimation of a current block, so that the accuracy of the extracted parameter is deteriorated and thus a problem of failing to generate an accurate estimated image is created.